1. Field of the Invention
The present invention relates to an image pickup device comprising a plurality of image pickup tubes or an image pickup board such as a CCD, MOS element, or the like as an image pickup element.
2. Related Background Art
An image pickup device, which photoelectrically converts image pickup light from an object using an image pickup element such as a CCD, and processes the converted electrical signal to display or record an object image, is known. As the image pickup element used as a photoelectric conversion element, elements having several hundred thousand pixels can be mass-produced according to the recent advance of IC techniques, and are popularly used in home video cameras. In order to meet a high-quality requirement of users, so-called "hi-vision" standard image pickup devices, which can assure higher image quality than the existing NTSC standard devices, are changing from an examination stage to a practical application stage. The hi-vision standard image pickup element requires a degree of integration as high as about two million pixels, and high-grade manufacturing techniques, resulting in a very expensive element. In addition, the signal read frequency is as high as several ten MHz, thus requiring high-grade circuit techniques.
Under these circumstances, a plurality of image pickup elements which are used in a conventional home video camera, and have several hundred thousand pixels, are arranged adjacent to each other on an image formation plane of an object image so as to obtain a high resolution corresponding to several million pixels. However, since the image pickup elements are normally sealed in packages, they cannot be closely arranged adjacent to each other on a single plane. Therefore, the above-mentioned expensive high-integration image pickup element requiring the high-grade techniques must be used.
In a conventional image pickup device, in order to increase the resolution, the high-integration image pickup element must be used, resulting in an expensive device, and necessity of difficult circuit techniques.
The number of pixels of an image pickup element, which can be formed in a unit image size, will undoubtedly keep on increasing according to future advance of semiconductor manufacturing techniques. However, when the number of pixels of the image pickup element is increased, the area per pixel is decreased almost inversely proportionally to the number of pixels. As a result, a light incident amount per pixel is decreased in proportion to the area per pixel, and sensitivity is decreased accordingly.
The same applies to an image pickup tube. That is, as the spot size of an electron beam is decreased, the resolution can be increased. However, the sensitivity is decreased for the same reason as described above.
In a conventional image pickup device, as described above, when the light incident amount is decreased, the sensitivity is decreased, and image quality of a dark portion (low-brightness portion) of an object is especially deteriorated. In addition, in order to prevent a decrease in S/N, the standard signal level of an image pickup element must be set to be relatively high.